Fire extinguishers (as I have mentioned elsewhere) are your absolute best friend in an active shooter situation. If you have no other option and you cannot escape or hide, they can be used to distract, to shield, and most importantly (and as the final resort) attack the skull of the gunman with extreme prejudice. And you don't want to stop until you see grey goop.

Did a training course with an FBI instructor a few years back who taught me that.